Do you have a pic that we can see what you are refering to? Not exactly sure the look you are going for.

As for using styro - not a problem. Make at least some of your flowers on toothpicks or wires and you will have no problem. If you don't want to us non-edibles/wire just pipe a pulled line of royal icing on the back of the flowers to stick into the foam. Styro uaually pierces easily

I would use SPS with the longer legs (the 7" legs). I would place a piece of styrofoam between the tiers to stick the flowers in, but because of SPS the styrofoam would not be the support system. So the density of the styrofoam wouldn't matter for any support issues.

If I were to remake the cake I would do the following:12 inch cake on at least a 16 inch board. Place an 8 inch SPS plate and pillars into the the 12 inch cake. Take an 8 inch styro dummy and tape that to the 8 inch SPS plate. I covered my styros in the same fondant as the cake. In your case the flowers are dense enough you won't see much peaking out but it may look nicer when they cut it etc. Place a piece of strong tape onto of the 8inch dummy. Place the 9 inch cake (that has a 6 inch SPS plate and pillar already pressed into it") on top of the 8 inch dummy. Then tape a 5 inch dummy on top of that. Then the 6 inch cake on top.

My biggest mistake is that my dummies were too small compared to the diameter of the cake sitting on top of them which made it a lot less stable. I have suggested a 1 inch difference, I think you could do 2 inches and be fine but anything more would be a bit wobbly.

For attaching the flowers. I used toothpics (as you can very clearly see in the pics which was disappointing to me). I would not do that today. If I had used the above tiers and separaters, I would have been able to just use melted chocolate and attach the roses straight to the separaters which I think would have looked much better in the long run. If you do use wires or toothpics, make sure they are not too long. It was very hard to get the flowers into the styrofoam without breaking them. I even prepped each hole but it was very difficult to get them pushed all the way in.

One great thing about the design you posted is it has filler flowers and leaves which means you won't have as much empty space as I did. Once you make the amount of flowers and leaves you think you'll need ...make more. I thought I had made way too many flowers and by the time I was done I used everyone and wished I had more.